The present invention relates to a thermal transfer recording sheet. More particularly, it relates to a thermal transfer recording sheet which can be used advantageously for color recording by OA terminals such as printer, facsimile, copier, etc., and for color recording of television images.
Various systems such as electrophotography, ink jetting and thermal transfer recording have been proposed and studied for the color recording, and thermal transfer recording is advantageous as compared with other systems in many respects such as maintenance of the apparatus, easiness of operation and inexpensiveness of expendables.
In the thermal transfer recording system, an image receiving material is placed on the ink-applied surface of a thermal transfer recording sheet applied with an ink containing colorants, and the backside of the said thermal transfer recording sheet is heated by a thermal printing head to transfer colorants to an image receiving material. The thermal transfer recording methods can be classified into two types, for instance, a thermofusion-type transfer recording system using a heat-fusible ink and a sublimation-type transfer recording system using an ink containing sublimable dyes.
In these types of thermal transfer recording system, however, the thermal transfer recording sheet is heated to a high temperature by the thermal head, so that if the base film of the thermal transfer recording sheet is not sufficiently high in heat resistance, the base film may be fused to the thermal head to cause improper running of the head relative to the thermal transfer recording sheet and other undesirable phenomena such as sticking, wrinkling or break of the thermal transfer recording sheet, making it unable to perform proper recording. For the improvement of heat resistance of the base film, it has been proposed to provide a protective film composed of various kinds of heat-resistant resin on the opposite surface of the base film to the colorant layer (Japanese Patent Application Laid-Open (KOKAI) Nos. 55-7467 and 57-74195). Also, for improving the running property of the sheet, it has been proposed to incorporate heat-resistant fine particles, lubricant, surfactant or the like substance in the protective layer (Japanese Patent Application Laid-Open (KOKAI) Nos. 55-146790, 56-155794 and 57-129789).
Recently, however, new problems have arisen in recording according to the said system. Because of accelerating of recording speed, a higher energy than used hitherto is given to the thermal head, resulting in a large load to the thermal transfer sheet, and it is difficult in the methods proposed in the above-mentioned Japanese KOKAIs to obtain a satisfactory running property of the thermal head relative to the thermal transfer recording sheet. Especially, in the case of thermal transfer recording sheet for the sublimation-type thermal transfer recording system using sublimable dyes, there is required a higher energy for recording than the case of thermal transfer recording sheet for thermofusion-type thermal transfer recording system using a heat-fusible ink, so that it is impossible to obtain the satisfactory running property of the thermal head relative to the thermal transfer recording sheet even if using a thermal transfer recording sheet treated with the proposed methods.
In running of thermal transfer recording sheet, it is ideal that the coefficient of friction between the backside of the thermal transfer recording sheet and the thermal head be kept constant irrespective of whether heating is applied or not, or the degree of heating. Since there are high-density portions and low-density portions in every image, the energy applied to the thermal head varies from part to part. If it is supposed that the coefficient of friction varies greatly depending on the degree of heating, then the tension exerted to the sheet varies from part to part in each image, that is, the sheet is pulled under high tension at a certain part, while almost no tension is exerted at another part. Under such a situation, running of the sheet and/or the image receiving material superposed therewith may deviate in the lateral direction, or the image receiving material may be traveled aslant, making it hard to obtain a clear and vivid image. Regarding coefficient of friction, there are known coefficient of static friction and coefficient of kinetic friction, and it is known that usually coefficient of static friction is greater than coefficient of kinetic friction. It is especially notable that coefficient of static friction during thermal printing is increased due to softening of the heat-resistant lubricating layer by heat, thereby encouraging occurrence of the sticking phenomenon such as mentioned above. The heat-resistant lubricating layer which the difference of coefficient of friction between heat-supplying state or no heat-supplying state is minimized is desirable. Further, a thermal transfer recording sheet which shows low in coefficient of static friction at heat-supplying state is required.
As the material of the practical heat-resistant lubricating layer having the desired running property, a crosslinked resin is prevalently used for the purpose of enhancing heat resistance. Specifically, UV-curing or heat-curing crosslinked resins have been proposed and practically used.
However, any of these crosslinked-type heat-resistant lubricating layers is rigid in its coating film, so that its touch with the thermal head is not uniform, causing nonuniform heat-conduction and roughening of the image formed.
Further, for producing these heat-resistant lubricating layers, since crosslinking treatment is essential, there are the serious problems relating to productivity. For instance, there is required a long-time heat-curing step or a specific UV-curing apparatus, and also difficulties are encountered in elevating the throughput rate.
Japanese Patent Application Laid-Open (KOKAI) No. 2-8087 proposes to incorporate an aminoalkyl-terminated polysiloxane and organic particles as lubricating material in the heat-resistant lubricating layer for enhancing the running property of the thermal head relative to the thermal transfer recording sheet, but this proposal was still unsatisfactory for realizing a practically satisfactory running property and storage stability.
These problems can be solved by using of a thermoplastic resin as the heat-resistant lubricating layer, but even in this case, there may arise the problem that the undesirable phenomena such as sticking tend to take place with the conventional synchronous transfer system.
As the result of the present inventors' earnest studies for overcoming the above-mentioned problems, it has been found that by conducting a thermal transfer recording using a thermal transfer recording sheet having a heat-resistant lubricating layer containing three specific components, while properly adjusting the relationship between the timing of feeding the thermal transfer recording sheet having the said heat-resistant lubricating layer and the timing of heat-supplying the thermal head, that is, by conducting a thermal transfer recording using a thermal transfer recording sheet having a heat-resistant lubricating layer containing a thermoplastic resin with a glass transition temperature of not less than 50.degree. C., an amino-modified silicone oil and a carboxy-modified silicone oil, especially conducting such transfer recording according to a thermal transfer recording system in which the feeding of thermal transfer recording sheet and heat-supplying of the thermal head are synchronized with each other, the thermal transfer recording sheet is not fused to the thermal head and the thermal head can maintain a good running property relative to the thermal transfer recording sheet even during high energy recording, making it possible to carry out efficient and uniform transfer recording, and to obtain a high-quality image free of roughness. The present invention has been attained on the basis of these findings.